Technical Field
The present invention relates to a flux-cored wire for Ar—CO2 mixed gas shielded arc welding which provides excellent welding workability in all-position welding when steel used for a steel structure or the like is welded and weld metal having excellent characteristics such as low-temperature cracking resistance, low-temperature toughness, and fracture toughness (hereinafter, referred to as CTOD).
Related Art
As a flux-cored wire used for gas shielded arc welding using steel as a material to be welded, for example, a rutile type flux-cored wire or a basic flux-cored wire is known. Welding using the basic flux-cored wire can reduce an oxygen amount of weld metal, and therefore the weld metal has excellent low-temperature toughness and CTOD characteristic. However, welding using the basic flux-cored wire has poorer welding workability in all-position welding than welding using the rutile type flux-cored wire, and therefore is not often used generally.
On the other hand, gas shielded arc welding using the rutile type flux-cored wire provides extremely excellent welding efficiency and welding workability in all-position welding, and therefore is applied in a wide range of fields such as shipbuilding, bridges, oceanic structures, and steel frames.
However, the rutile type flux-cored wire is obtained by filling a flux mainly including a metal oxide such as TiO2 into a steel outer skin, and therefore weld metal has a large amount of oxygen and does not easily obtain low-temperature toughness.
In addition, in these flux-cored wires, an amount of diffusion hydrogen is larger than that in a solid wire due to moisture included in a raw material of the flux or moisture absorption while the wire is stored. Therefore, there is a risk of low-temperature cracking of weld metal. It is necessary to perform preheating at about 100° C. when a thick steel plate is welded. This reduces a welding efficiency.
Various developments have been performed for a rutile type flux-cored wire of low-temperature steel. For example, JP 2009-61474 A discloses a technology for obtaining weld metal having excellent low-temperature toughness by reducing an oxygen amount of the weld metal while an amount of slag acting on welding workability is maintained by adding an alloy component which changes into a slag component during welding. However, in the technology described in JP 2009-61474 A, a shielding gas is CO2, and therefore the oxygen amount of the weld metal increases, sufficient low-temperature toughness or CTOD value is not obtained, and low-temperature cracking resistance is not taken into account even though high-temperature cracking resistance is secured.
JP 2008-87043 A also discloses a technology for obtaining weld metal having excellent low-temperature toughness. However, MgO added for reducing an oxygen amount of weld metal makes an arc unstable during welding, generates a large amount of spatters, and makes welding workability poor. In the technology disclosed in JP 2008-87043 A, low-temperature cracking resistance is not particularly taken into account.
JP 2009-248137 A discloses a rutile type flux-cored wire which provides excellent workability in all-position welding and weld metal having excellent low-temperature toughness. However, in the technology disclosed in JP 2009-248137 A, stabilization of low-temperature toughness is not sufficiently studied, and therefore an excellent CTOD value is not obtained disadvantageously.
JP 9-277087 A discloses a technology for obtaining excellent low-temperature toughness both in welding and in a heat treatment after welding by limiting contents of Nb, V, and P in a wire. However, also in the technology described in JP 9-277087 A, workability in all-position welding or a CTOD value is not sufficient disadvantageously.